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Keep in: To retain or maintain something within a confined space or boundary. 
          He always keeps in his emotions and rarely shows them. 

Keep on: To continue doing something. 

         Don't give up, just keep on trying. 

Keep up (with) : To maintain the same pace or level as.

          It's hard to keep up with the latest technological advancements. 

Knock down : To demolish or destroy. 

         They plan to knock down the old building and construct a new one.